You need to install a hot-oil system. Maybe you are increasing production or perhaps installing a new reactor, evaporator, dryer, press or something else entirely. Your existing heater is too far away, too small, or too old and inefficient to handle the increased heat load. So, you face the age-old question: How do you go about choosing between an electrically heated and a fuel-fired heater?
The answer: “It depends.”
Historically, electric has been the preferred energy source for smaller systems (less than 0.5 million BTU/hr) primarily because its higher operating cost was more than offset by lower installation costs. Fired-heater manufacturers did not offer products in that range because they were simply not competitive. Above 1 to 2 million BTU/hr, operating costs for fired heaters (mostly natural gas in North America) were low enough to offset the higher cost of installing a natural gas line and exhaust stack.
Recent wild price swings in energy costs, particularly in the natural gas market, have changed the equation. Stung by the loss of large industrial users, large power utilities reportedly have offered low electric rates to their remaining industrial customers. As a result, some fired-heater users have evaluated installing electric heaters for loads as high as 10 million BTU/hr. While many such projects were quickly abandoned when natural gas prices dropped, they illustrate the potential that energy-price swings can have on equipment selection.
Most of the current forecasts from the Department of Energy’s Energy Information Administration (EIA) predict stable domestic dry gas production with increases in supply primarily provided by liquefied natural gas (LNG) imports. The unresolved security issues around the LNG unloading stations could still change the supply-side projections. In addition, the final outcome of the legislation to reduce carbon emissions will affect demand because many of the solutions to reduce carbon emission involve natural gas. For example, building natural-gas-fired power plants, which on an installed basis are cheaper than coal or nuclear, provides a path to reduced power-generation emissions that has relatively low financial risk. Natural gas also is touted as a potential fuel for cars. In short, the only certainty is that the current cost advantage of natural gas may be reduced or even eliminated.
So, the choice between a fuel-fired or electric heater above the 0.5 million BTU/hr output range should be evaluated based on a multiple-year financial analysis of the installation and operating costs for each type of heater. It also should include a “what if” case that utility costs will be equal.
Consider Installation RealitiesOn the electric side, include the cost of increasing the building’s electrical service even if the power requirements can be handled by the existing wiring. Electric demand can be difficult to project with any accuracy given that it is a more versatile utility than natural gas, fuel oil or propane (propane powered computers, anyone?). Few things arouse management curiosity more than having to revisit building power projections and the cost of increasing them after the new electric heater is installed.
Fired-heater maintenance projections should include an annual service call by factory technicians to check the combustion system and related safety switches. Burners may require replacement every couple of years, and the refractory may have to be recast.
Fired heaters have a way of attracting more attention from property insurers and local authorities than electric heaters. Insurance companies offer extensive guidance for the installation and operation of any heater where the energy source involves burning something.
Hard “coke” deposits can form in heaters when the fluid becomes oxidized, usually due to improper operation of the expansion tank. While this occurs in both types of systems, electric elements are more prone to failure than heater tubes if coking occurs, so include money for element replacement in the projected costs.
Consider Application and Process ConditionsFor a continuous single-user process where the heater output equals the average heat requirement, a straightforward “installed vs. operating” cost analysis will yield the best result when comparing fired and electric options.
If the application will be run in a batch mode or intermittent product runs of short duration, consider some additional questions.
Will Startup Time Be an Issue? In a fired thermal fluid heater, the refractory, heater tubes and exhaust stack all have to be brought up to temperature at a controlled rate to minimize thermal stress. In addition, the fluid itself can be damaged in the radiant section of the heater if the burner reaches high fire before the rising fluid temperature has resulted in enough of a viscosity reduction to initiate turbulent flow.
In an electric thermal fluid heater, the elements are immersed in fluid flowing inside a pipe, so the only thermal mass in the system is the pipe that encases the heating element. Because the maximum heat flux in an electric heater is less than the average heat flux in a fired heater, the element surface temperature (film temperature) remains relatively close to the fluid’s bulk temperature, which minimizes any thermal degradation. As a result of these features, electric heaters can be brought up to temperature more quickly than fired heaters.
Is Precise Temperature Control Required? Electric heaters can be equipped with SCRs, which provide infinite turndown. Because of the inherent time lags of combustion controls, blowers and burners, gas heaters typically do not respond as quickly or as precisely.
How Many Individual Applications? With multiple heat users, the fired vs. electric analysis is slightly more complicated. You’ll need to evaluate the cost of a central heater (almost always fired, based on the required heat load) vs. several stand-alone electric units.
- Installation Costs. If a failure of the central heater could cause a significant process problem -- for example, reactants gelling to a solid in an expensive glass-lined reactor -- consider installing a backup heater or split the load between two smaller electric units. Alternatively, secondary pumps at the users may be required to improve temperature-control accuracy. Stand-alone units may need to be installed away from the user or in a protected enclosure if the process area is prone to spills.
- Maintenance Costs. Proper piping design ensures that one line is open to the expansion tank at all times. Because most hot-oil-system mishaps are due to expansion tank malfunctions, having a single tank to check is an important advantage of a fired central system. By contrast, with individual electric systems, each stand-alone unit will have its own expansion tank, fluid level, safety circuits and instrumentation to maintain. In general, these require approximately the same maintenance effort as a large central system. Also, stand-alone electric units are often packaged in metal enclosures that protect the components from dirt and product spills but can also hinder access to components, particularly pumps.
- Is Cooling Required? If all of the end users require cooling in approximately the same temperature range, central heating and cooling loops make sense. All that is required at each user is a mixing valve and, possibly, a secondary pump loop to improve temperature control. If only some of the end users require cooling, a secondary pump loop with a heat exchanger (oil-to-water or oil-to-air) can be installed. By contrast, if there are widely varying temperature requirements for heating and cooling, stand-alone electric systems have an intuitive appeal. Some stand-alone units have built-in refrigeration systems so cooling water connections are not necessary. This is a definite advantage in plants where a central chiller system is not available.